Starting point for endeavour in which maybe four succeeded, reportedly? (6,3)
I believe the answer is:
square one
'starting point for endeavour' is the definition.
'maybe four succeeded reportedly?' is the wordplay.
'maybe four' becomes 'square' (four is a square number).
'succeeded' becomes 'won' (I've seen this before).
'reportedly?' shows a homophone (sound like) (I've seen 'reportedly' mean this).
'won' is a homophone of 'one'.
'square'+'one'='SQUARE ONE'
'in which' is the link.
(Other definitions for square one that I've seen before include "Back to . . . . . . . . . - at starting point with no progress made" , "back here after failure" .)